Screw compressors have been used as compressors for compressing a refrigerant or air. For example, Japanese Patent Publication Nos. 2004-316586 and H06-042474 disclose a single screw compressor including a single screw rotor and two gate rotors.
The single screw compressor will be described below. The screw rotor is substantially in the shape of a round column, and a plurality of helical grooves are formed in an outer peripheral surface thereof. Each of the gate rotors is substantially in the shape of a flat plate, and is arranged laterally adjacent to the screw rotor. The gate rotor includes a plurality of rectangular plate-shaped gates which are radially arranged. The gate rotor is arranged with an axis of rotation thereof perpendicular to an axis of rotation of the screw rotor, and the gates mesh with the helical grooves of the screw rotor.
The screw rotor and the gate rotors of the single screw compressor are contained in a casing. Fluid chambers are formed by the helical grooves of the screw rotor, the gates of the gate rotor, and an inner wall surface of the casing. When the screw rotor is rotated by an electric motor etc., the gate rotors are rotated by the rotation of the screw rotor. The gates of the gate rotors move relatively from start ends (ends through which a fluid is sucked) to terminal ends (ends through which the fluid is discharged) of the meshed helical grooves, thereby gradually reducing a volume of the fluid chamber which is completely closed. In this way, the fluid in the fluid chamber is compressed.
As disclosed by Japanese Patent Publication Nos. 2004-316586 and H06-042474, the screw compressor includes a slide valve for controlling a capacity. The slide valve is arranged to face an outer peripheral surface of the screw rotor, and is slidable in a direction parallel to the axis of rotation of the screw rotor. The screw compressor includes a bypass passage for communicating the fluid chamber in a compression stroke with a suction side of the compressor. When the slide valve moves, an area of an opening of the bypass passage in an inner peripheral surface of a cylinder in which the screw rotor is inserted varies, and a flow rate of fluid returned to low pressure space through the bypass passage varies. As a result, a flow rate of fluid which is finally compressed in the fluid chamber and discharged therefrom varies, and a flow rate of fluid discharged from the screw compressor (i.e., an operating capacity of the screw compressor) varies.